GM rear wheel drive transmissions were cast with a number of different bolt patterns to mate to various engines. There are three common bolt patterns. In many cases, a transmission with one bolt pattern will not mate to an engine with a different bolt pattern without the use of an adaptor.
Above is the "Chevy" Bolt Pattern. The high center peak at the top is it's most noteable feature. Transmissions with this bolt pattern will mate to most Chevy engines (most front wheel drive engines used a different bolt pattern not illustrated here).
Above is the "BOP" (Buick-Olds-Pontiac) Bolt Pattern. The depressed flat at the top is it's most noteable feature. Transmissions with this bolt pattern will mate to most Buick, Cadillac, Oldsmobile and Pontiac engines (most front wheel drive engines used a different bolt pattern not illustrated here).
The third common bolt pattern is what is often refered to as the "Dual" Pattern bolt pattern. Or sometimes called the "Universal" pattern. Transmissions with this pattern will bolt up to engines with either of the two patterns above eliminating the need to use an adaptor plate.
The lower two bolts and the locating dowels are in the same locations for both the "Chevy" and the "BOP" bolt patterns. The locations of the upper four bolts is different. The "Dual" takes advantage of this and provides for mating to either pattern.
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